So I just came back from a trip downtown and handled the Mi 4 for the first time - spending about 30 minutes with it in the store.
First impressions were it's a bit unwieldly to hold! It's a strange kind of hybrid design where it's almost exactly the angular shape of the iPhone 5 and 5s (just bigger) but then has a slight curvature to the back near the edges. I think they did this to try to mask the thickness of the device - it is noticably chunky in the hand - especially since I was also playing with the Find 7 which is much more pleasant to hold due to the slimmer edges, if not easy to operate with one hand. It feels like it gets too fat, too soon as opposed to the gentler curves on the F7 and M7.
I would say the screen size is perfect (5-inch) for my one-handed use with relatively slim bezels. If it was a millimetre or two thinner, or had a better all-round curvature akin to my M7, it would be my perfect form factor. Nit-picking but I much prefer matte black if it has to be plastic (the Find 7's carbon-like rear cover is really nice).
As to the performance, it was running MIUI and was super-slick as you'd expect with 3GB RAM, Snap 801 and the other top-line specs. No idea about the battery life but with the screen at 1080p it isn't pushing crazy numbers of pixels around and it's packing 3080mAh. The camera seemed good although I could only zoom in on images on the phone rather than eyeball them in Photoshop. Screen was nice and no notable downsides when sat next to the F7, my 5s and M7.
The biggest problem with the 16GB model is the 16GB - it's too small thesedays, especially when there's barely 12GB of usable space. The sales people tried their best to sell it to me but I wasn't tempted as this was a deal-breaker. I asked for the 64GB version and in black but it's not yet available. Had they had that model, I would have bought one there and then despite the slight chunkiness. At 2,499CNY for the 64GB model (~US$400) it's quite the bargain. Conversely, I'd see 1,999CNY (~US$350) as a waste and be consistently annoyed at having to juggle apps with the 16GB model.
It still might be the successor to my M7 but I'm still open to other 4.7-5.0-inch devices. Maybe the Z3 Compact will lose the slab-shape